This invention concerns a new device for regenerating mixed beds of ion exchangers used for the desalination of water or aqueous solutions deriving from industrial processes (process solutions) and the method of carrying out this regeneration.
The technique of desalination by means of mixed beds of ion exchanger resins, described for the first time in 1951 (U.S. Pat. No. 2,578,937), is now widely used for the production of very low ion content water: among the most important applications are the production of steam-boiler feed water and water for the electronic and pharmaceutical industries.
Another application of mixed bed processes concerns the desalination of process solutions containing nonionic organic compounds, as, for example, molecules of pharmaceutical or foodstuffs interest. What characterizes mixed bed processes is the fact that water or the solution to be desalinated is percolated through an intimate mixture of a cation exchanger and an anion exchanger.
Mixed bed treatment allows the reduction of the residual saline content of treated water to quite lower levels as compared with desalination through separate beds of two ion exchangers; in fact, whereas in the case of separate beds, the fraction of ions removed is limited by the equilibrium value corresponding to the maximum degree of regeneration of the ion exchangers and hardly exceeds 99%, in the case of a mixed bed there are no theoretical limits to the fraction of ions removed.
Furthermore, whereas during the treatment, for example, through separate beds of cationic and anionic exchangers connected in series, the pH of the treated solution falls to very low levels in the column containing the cationic resin, in the mixed bed pH remains near to neutral. This characteristic allows, for example, the desalination of solutions of pH sensitive molecules.
Opposed to these advantages, the regeneration of mixed beds after use is unfortunately till now much more complex and costly than with separate beds, because the cation exchangers and the anion exchangers must be separated before regeneration, which is carried out with acids and bases respectively and then they must be homogeneously remixed after regeneration.
Separation and regeneration of exhausted mixed beds has been described for the first time in U.S. Pat. No. 2,771,424 (1956). A monography of recent processes was reported by B. Coulter, Ultrapure Water, November 1987.
In all regeneration processes, the resins are separated by hydraulic classification by utilizing the different densities and granulometries of the two exchangers.
Once separated, the resins can be regenerated in the same column which contained the mixed bed (internal regeneration) or one or both of them can be transferred into one or more different columns where regeneration is carried out (external regeneration); they are then mixed in a special mixer (or even in the column used for the regeneration of the cation exchanger) and then transferred into the column used for the mixed bed. Another possibility consists in transferring said resins, after regeneration, into the column used for the mixed bed and mixing them therein.
The external regeneration procedure requires a much more complex plant, and is therefore normally used only for the final desalination of water to be fed to steam-boilers in thermoelectric or thermonuclear power stations.
The most widely used process for small or medium sized units is the internal regeneration. In this latter case the reagents for regeneration of the anion and the cation exchangers enter the column from the top and bottom respectively, either simultaneously or at different times (regeneration of the anionic exchanger is normally carried out first), while the exhausted regenerating solutions are collected from the same discharge line provided with devices (strainers) able to retain the portion of resin situated near the interface between the resins.
This system is less costly but has two significant disadvantages:
1) the interface between the resins must be exactly maintained at the level of the discharge line, otherwise a part of the anionic exchanger will be saturated by the acid used for the regeneration of the cationic exchanger or, vice versa, a part of the cationic exchanger will be saturated by the base used for the regeneration so the anionic exchanger.
This fact implies that it is not possible to handle a mixed bed with quantities of cationic resin different from those of the original design and that each variation of volume of the cationic resin, either due to the normal swelling occurring during regeneration or to a possible loss of resin, will have negative effects on the following performance of the bed;
2) even if the level of the interface is maintained at the level at which the discharge line has been installed, there will always be a certain mixing between the two regenerating liquids or between one regenerating liquid and the barrier water, in a substantial volume around the interface between the resins.
Both problems 1 and 2 cause incomplete regeneration of that portion resins which is near to the interface; it means that a part of the anionic resin will be saturated with the cationic resin regeneration liquid and vice versa. This implies a lower exchange capacity in the regenerated resins at equal volumes and consumption of regenerating liquid; furthermore, the portion of saturated anionic resin will release sulphate or chloride ions and the portion of saturated cationic resin will release sodium ions, so affecting the quality of water produced in the successive desalination process (see e.g. G. J. Crits, Ion Exchange, Technology of Mixed Beds, Ultrapure Water, November 1984).
These disadvantages can be reduced by introducing into the mixed bed, other than the two ion exchange resins, an inert separator consisting, for example of inert small beads with an intermediate density between those of the two resins.
During hydraulic classification, the separator positions itself between the two resins distancing them from the zone where mixing of the two regeneration reagents takes place.
In this way, the partial saturation of the anionic resin is reduced as well as the criticality of the level of the cationic resin, but the desalination capacity per unit volume of bed will be inferior because the inert separator occupies a portion of the column volume.
On the contrary, external regeneration eliminates all of the problems related to imperfect separation of the regenerating reagents in such way assuring greater exchange efficiency, an improved degree of purity of the treated water and better repeatability of the desalination process: a recent mixed bed process with external regeneration is, for example, described in U.S. Pat. No. 4,472,282.
On the other hand, transferring the resins involves, as has already been pointed out, complex equipments and troublesome handling of the same and long overall regeneration times, thus making this method economically feasible only for the treatment of big volumes of water, already with a very low salt content.
Consequently, the equipment complexity is justified only for big plants and such operational work and long regeneration times are only acceptable when infrequent regeneration cycles (see e.g. B. L. Coulter, art. cit.) are required.
Another critical point in all mixed bed processes is the homogeneous mixing of the ion exchange resins once the regeneration is carried out. It is well known that the quality or water produced and the working capacity of the bed, largely depend on the quality of the mixing which must be as homogeneous as possible (see e.g. E. G. Baeva et al., Development of a System for Mixing Ion Exchangers, Teploenergetika, 1968).
In the devices known in the state of the art, mixing is always obtained by fluidizing the bed by counter-washing with water and then blowing air from the bottom of the column. This method is also applied in external regeneration units, with the sole difference that in the latter case, mixing is sometimes effected in a special apparatus instead of in the column dedicated to the mixed bed.
Mixed beds obtained by this procedure can be, and usually are, lacking in homogeneity: in general, the upper part of the bed consists almost exclusively of the lighter anionic resin and the lower part almost exclusively of the cationic resin (data concerning lack of homogeneity in mixed beds are given by Baeva and S. Fisher, Trouble Shooting in Mixed Bed Ion Exchange, Ultrapure Water, July-August, 1992). Only the central portion contains both resins mixed in quantities approaching the optimal ratio; however, if, for example, transparent columns are used, a simple optical analysis shows that even in this portion, homogeneity is not optimal: relatively large portions (of the order of 0.5 L in a 40 L bed) in which cation exchangers prevail, alternate with zones of the same size in which anion exchangers prevail.
In conclusion, available mixed bed techniques at the current state of the art, have disadvantages as compared to the conventional treatment with two or more separate beds, which are tied on the one hand to greater plant and operational complexity (above all if external regeneration is used) and on the other hand to high handling costs and, as a consequence, are competitive for the production of ultrapure water or of process solutions with very low ionic content, only in the case solutions with already very low saline content, usually lower than that found in well water.
On the other hand, separate bed processes are not normally usable to produce water or process solutions with a conductivity of less than 0.5 xcexcS/cm. Consequently, the production of water with very low conductivity (that is less than 0.5 xcexcS/cm, preferably less than 0.25 xcexcS/cm or even 0.08 xcexcS/cm, such as for example for thermonuclear plants) normally requires two treatment steps, in which only the second is carried out with a mixed bed.
Furthermore, internal regeneration processes are normally used for small mixed beds and as has been previously stated, they are rather unsatisfactory, even for the quality of the deionized water produced in each successive phase.
It is to the subject of this invention a new device for the regeneration and mixing of ion exchangers resins in a mixed bed and method for operating it. This handling is much simpler in comparison to the units with external regeneration of the present state of the technology, it maintains all of their advantages and even increases their performance, in particular thanks to the greater homogeneity of the mixed bed obtained by said method.
A further object of this invention is the method described below for the preparation of a mixed bed of ion exchangers, characterized by great homogeneity. This process does not require the use of air for mixing the resins and is applicable to units with external regeneration.
The simplicity and versatility of the plant object of this invention, make it usable even for small or medium sized applications and even in processes which require frequent regeneration, for the first time rendering possible, and economic, to obtain purified water with a purity degree similar to those obtained by the best units with external regeneration, even when starting from well-water or even from sea-water.
The mixed bed obtained by the method and the plant object of this invention, is furthermore able to reduce inorganic and organic ionic impurities to extremely low levels in aqueous solutions of neutral organic products (for example, molecules of pharmaceutical interest and their intermediates or sugar solutions or food products).
The scope and advantages of the device and the method according to the invention are reached with the characteristics listed respectively in independent claims 1 and 6. Advantageous applications of the invention appear in the dependent claims.
Substantially, according to the invention, two columns are foreseen: a first treatment column containing the mixed bed of ion exchange resins, in which the cation exchange resins are regenerated after the treatment/desalination process, and then a second column into which the anion exchange resins are transferred and regenerated, to be then reinserted at the bottom of the first column where they rise through the cation exchange resins present therein, intimately mixing themselves with these, to give a reconstituted homogeneous mixed bed.